One of the methods of improving the smoothness of paper/paperboard during the calendering operation is to pass the paper/paperboard through one or more heated nips which are at a temperature higher than the temperature of the web. The surface of the paper/paperboard that is to be finished is pressed against the heated roll. The applied heat raises the surface temperature of the paper/paperboard to the glass transition temperature, which causes the fibers to soften and conform to the surface of the roll. Moisture (i.e., water or steam) can also be added before the nip to the surface that is treated to further lower the glass transition temperature. Thus, gradients in the temperature and moisture level tend to lower the glass transition temperature preferentially on the external surfaces of the paper/paperboard and allow the sheet to achieve a desired smoothness without significant reduction in caliper.
Temperature gradient calendering is a known method of enhancing the smoothness of paper/paperboard without sacrificing caliper. However, high temperature alone cannot yield the degree of smoothness that is required for some grades. Moisture is essential for the smoothness development. This can be observed in FIG. 1 where Hagerty smoothness of bleached board calendered in a steel-steel nip at 350.degree. F. is shown as a function of the apparent density of the final product. It should be noted that within the range of applied niploads, calendering without moisture addition can only result in a minimum smoothness of 180 units while the sheet calendered with addition of moisture to raise the total moisture content by 0.5% can attain a smoothness of 130 Hagerty units.
The prevalent method of moisture addition for most board and bristol grades is by using waterboxes on at least one calender stack. This process usually consists of overdrying the sheet to obtain a flat moisture profile of 1-2% and then adding water using waterboxes. The moisture pickup is typically greater than 10-12% of the conditioned weight of the paper/paperboard and can sometimes be as much as 15-18% of the conditioned weight of the paper/paperboard. One purpose of overdrying is to correct for any moisture nonuniformity. Overdrying the sheet can only be achieved by running the machine slower, thereby reducing production. In addition, waterboxes tend to cause several operational problems, including breaks and difficulty during the threading process. The high level of moisture added with the waterboxes in a multiroll calender stack makes it necessary to employ drying means so that the reel moisture can be maintained at 5-8%. This drying is typically accomplished using intercalender dryers which are usually run without any supporting fabric. Following this drying, the paper sheet is passed through a multiroll stack with several nips to be subjected to increasing pressure, whereby the web develops good smoothness and high density. One of the advantages of waterboxes is that the water applied can incorporate other functional additives, such as dyes, lubricants, binders such as starch, and film formers such as polyvinyl alcohol.
Addition of the moisture in such a way that only the surface is moistened to a lower glass transition temperature can result in a paper/paperboard having high smoothness and low density. An application similar to this has been suggested in U.S. Pat. No. 5,378,497, wherein the moisture is applied before the calendering nip using a metering size press. The '497 patent further discloses that the smoothness developed is irreversible and is useful in operations that involve rewetting the sheet, such as coating/tinting. The metering size press method applies a lower amount of moisture than does the waterbox but cannot correct profile nonuniformities. In addition, installation of a metering size press involves significant cost and machine reconfiguration.
In addition to the foregoing, the temperature gradient calendering of moistened paper/paperboard that is surface sized is impeded by starch and associated fibers picking and sticking on the heated roll. This problem tends to be aggravated at the higher niploads and temperatures required for achieving high smoothness levels. The sticking and fiber pulling are caused by the starch or other binders that are added in the size press and lead to runnability problems.